Clathrate hydrates constitute a class of solids
in which the guest molecules occupy, fully or partially, cages in host structures
made up of H-bonded water molecules. The usually unstable empty clathrate
is stabilised by inclusion of the guest species. In case of guest molecules
which are gaseous at ambient conditions the resulting clathrate hydrate
is often called a gas hydrate. These compounds are interesting for several
reasons: much could be learned about water-water interactions in these topologically
rather complex systems, especially if one could follow the pressure dependency
of the host structure over a wide pressure range. Likewise, much could be
learned about the guest-host interactions in a wide range of guest species
from noble gas atoms to large (and polar) organic molecules. Clathrates
are believed to occur in large quantities on some outer planets binding
gas at fairly high temperatures, which is an interesting issue for planetologists.

This is, however, far from being only of academic interest: The petrol industry suffers from the nuisance of hydrocarbon-hydrates blocking gas pipelines in arctic regions, yet are beginning to show interest in the giant natural methane hydrate deposits on the deep ocean floor and in permafrost regions. Rapid methane hydrate decomposition triggered e.g. by earthquakes may lead to catastrophic submarine landslides producing tsunamis of a giant size. Moreover, the decomposition of methane hydrate may well affect the world's climate; there are indications for climatic changes provoked by gas hydrate decomposition in geological records. Yet this is not the only area of interest in the field of gas hydrates: They may be used as a cheaper alternative of gas storage and transport as compared to liquefied gas, and gas hydrates may be used in the desalination of sea water. Moreover, Japanese companies have suggested in the early 1990s a deep sea deposition of CO2-clathrate to remove this greenhouse gas from the atmospheric cycles and, recently, large CO2 sequestering programmes were started in the US.

The cage filling of gas hydrates is governed by
thermodynamics. In the late 1950s a statistical thermodynamic theory was
developed which allowed the prediction of stability and gas filling for
gas hydrates. The theory is based on the following main assumptions:
1. The free energy of water structure is independent of the guest occupation;
i.e. no lattice distortion.
2. Each cavity contains only one guest entity.
3. No interaction between guest entities, i.e. enclathration is described
as Langmuir adsorption.
4. No quantum effects.

Our early work on gas hydrates

Early on, our interest in the high pressure phases of ice brought us into contact
with gas-filled ices as well as clathrate hydrates. In 1988 we were the first
to describe "stuffed" ice, a helium-filled high pressure ice with
a water topology identical to ice II. In 1992 we succeeded for the first time
worldwide to produce pure argon clathrate hydrate starting from ice Ih close
to the melting point, a method which was reinvented a few years later by other
groups. Using this material, we were again the first to determine the crystal
structure of a gas hydrate under in situ conditions.

Air hydrates in ice sheets

Snow is a material with very low density. Upon densification some air is contained
in firn and eventually closed off in the final glacier ice. In polar ice sheets
these air bubbles experience an increasing mechanical pressure by the overburdening
snow and ice. Upon snow accumulation the underlying ice is pushed into deeper
and deeper parts eventually reaching pressures where ice and air will transform
into a solid crystalline compound, air hydrate. In cooperation with the Alfred-Wegener-Institut
in Bremerhaven, our group has investigated the fractionation processes of oxygen
and nitrogen during this transition and has revealed the complicated crystallisation
process taking place in several steps.

Octahedral (left) and tetrahedral (right) air clathrate hydrate crystals from the NGRIP ice core (central Greenland ) found in a depth of 1271 m and 1378 m, respectively.

Crystallographic structures

Two main distinct structure types exist for gas hydrates, both with cubic symmetry:
type I with spacegroup Pm3n and type II with spacegroup Fd3m. The nomenclature
is from von Stackelberg who studied gas hydrates in the 1940s and 1950s.
Our group has a long-standing record in the structural determination of gas
hydrates under in situ conditions. Such work is very suitable to the check assumptions
underlying thermodynamic theories mentioned above. These theories are the basis
for computer programs widely used in chemical engineering and geosciences for
predicting gas hydrate composition and stability. Our work (1997) has established
for the first time that certain gas hydrates may contain more than one molecule
in their cages. Likewise, we have found evidence for a pressure-dependent distortion
of the water host-lattice which was assumed in earlier thermodynamic theories
to be negligible.

The crystal structure of air hydrate. The red-white wire-fram shows the water host lattice forming two types of cages in which the oxygen (blue) and nitrogen (green) atoms are located. The resulting structure has cubic symmetry and belongs to von Stackelberg's type II.

Microstructure

Gas hydrates have a number of unusual properties. Among them is their remarkable
microstructure. We have found in 2000, using field-emission scanning electron
microscopy, that all investigated gas hydrates may form single crystals sized
from a few microns to a few tens of a micron and showing a nanometric porous
microstructure. Gas hydrates are apparently the only known material in which
regular pore structures are formed spontaneously within quite perfect single
crystals. Dense (i.e. non-porous) hydrate crystals also exist. Whether dense
or porous hydrates are formed depends on the formation conditions. Meanwhile
we have, partly in collaboration with GEOMAR and the University of Bremen, investigated
the microstructure of a large number of natural gas hydrates from the ocean
sea floor and sub-permafrost regions.

Single crystal of methane hydrate with facettes and a nanometric
porous microstructure (left: overview, right: detail). The pore diameter
in this example typically is 200 nm.

Formation kinetics

The formation process of gas hydrates is quite complicated and not understood
in detail. The gas molecules have to be built into the host lattice during the
growth process. Starting from water with its well-known low-solubility for most
gases, the growth process is quite slow. Starting from ice can considerably
speed up the process. Quantitative studies of the growth kinetics can give detailed
insights into the growth mechanism. We have undertaken a large number of in-situ
diffraction experiments to study the growth kinetics. In combination with formation
runs interrupted at various stages of the process where samples were recovered
and investigated by electron microscopy we were able to establish a multi-stage
model for the gas hydrate growth. Each stage has its characteristic time-dependency
with an overall reaction rate slowing down as the formation proceeds.

Decomposition kinetics

Gas hydrate decomposition can be as complicated as the formation process. In
particular, in a temperature window below 0°C the gas hydrate decomposition
could be slowed down by several orders of magnitude, the so-called anomalous
preservation. This effect is not well understood. By a detailed diffraction
study of the decomposition kinetics and an analysis of the formed defective
ice we can suggest that the onset of anomalous preservation is caused
by the annealing of defective ice. The annealed ice forms an effective diffusion
barrier for gases hindering the out-diffusion of the gases released upon decomposition
and providing the chemical activity at the gas hydrateice interface to
stabilize the hydrate.

Reference:

Inelastic neutron scattering and molecular dynamics

Gas hydrates show unusually small thermal conductivities with temperature dependencies
more closely resembling glassy materials than crystalline ice. These properties
can be understood by the interaction of localized low-frequency guest modes
with acoustic phonons of the host-lattice. We have studied these interactions
by means of inelastic neutron scattering and molecular dynamics simulations.
On varying the guest entities, we found that the guest-host coupling varied
remarkably in strength. Likewise, pronounced differences were found in the guest
modes of molecules quite similar in size and mass (like N2 and O2). This led
us to the conclusion that "gas hydrates are individuals".

Ab-initio work

Despite the simple chemical composition of water, the interactions of its molecules
are quite complicated and still far from being fully understood. Water molecules
can form a large number of clusters in which they are hydrogen-bonded. In these
clusters, molecules can either accept or donate a H-bond. We have undertaken
ab-initio studies to better understand the structure and stability of water
clusters, in particular the so-called Buckyball-clusters which topologically
resemble the cages in clathrate hydrates. Interestingly, we have found a new
classification scheme for H-bonds, which could help in predicting the relative
stability of water clusters without the necessity for expensive ab-initio calculations.
This classification scheme is presently used to analyse the molecular rearrangements
of water molecules at the surface of gas hydrates.

a)

b)

c)

Certain Buckyball water clusters are topologically identical to cages in clathrate hydrates. The three types of cages forming cubic clathrate hydrates of type I and II consisting of 5- and 6- membered rings of H-bonded water molecules are shown here. (a) 512 cage or pentagondodecahedron  small cage of both the type I and type II clathrate structure, (b) 51262 cage or tetrakaidecahedron  large cage in a type I structure, (c) 51264 cage or hexakaidecahedron  large cage in a type II structure.

Reference:

Chihaia, V., S. Adams and W. F. Kuhs
(2004)Influence of water molecules arrangement on the structure
and stability of 512 and 51262
Buckyball water clusters. A theoretical studyChem. Phys.297, 271-287.